Reference Guide

826 |sFlow
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Figure 45-1. sFlow Traffic Monitoring System
Implementation Information
The Dell Networking sFlow is designed so that the hardware sampling rate is per stack unit port-pipe and
is decided based upon all the ports in that port-pipe. If sFlow is not enabled on any port specifically, then
the global sampling rate is downloaded to that port and is to calculate the port-pipe’s lowest sampling rate.
This design supports, then, the possibility that sFlow might be configured on that port in the future.
Back-off is triggered based on the port-pipe’s hardware sampling rate.
For example, if port 1 in a the port-pipe has sFlow configured with a 16384 sampling rate while port 2 in
the port-pipe has sFlow configured but no sampling rate set, FTOS applies a global sampling rate of 512 to
port 2. The hardware sampling rate on the port-pipe is ten set at 512 because that is the lowest configured
rate on the port-pipe. When a high traffic situation occurs, a back-off is triggered and the hardware
sampling rate is backed-off from 512 to 1024. Note that port 1 maintains its sampling rate of 16384; port 1
is unaffected because it maintain its configured sampling rate of 16484.
To avoid the back-off, either increase the global sampling rate or configure all the stack-unit ports with the
desired sampling rate even if some ports have no sFlow configured.
Important Points to Remember
• The FTOS implementation of the sFlow MIB supports sFlow configuration via snmpset.
• Dell Networking recommends that the sFlow Collector be connected to the Dell Networking chassis
through a stack-unit port rather than the S5000 Management Ethernet port.
• On the S5000, sFlow sampling is done on a per-port basis.
• FTOS exports all sFlow packets to the collector. A small sampling rate can equate to a large number of
exported packets. A backoff mechanism will automatically be applied to reduce this amount. Some
sampled packets may be dropped when the exported packet rate is high and the backoff mechanism is
about to or is starting to take effect. The dropEvent counter, in the sFlow packet, will always be zero.
sFlow Collector
sFlow Agent
Switch/Router
Switch ASIC
sFlow Datagrams
Poll Interface
Counters
Interface
Counters
Flow Samples